DOUBLE MONSTERS IN FISHES 59 



possibly to be interpreted as concrescence. Similarly 

 the Knopf regions come together to form a single median 

 tail-bud, which completes the embryonic axis and is to 

 be considered as the final step in this very limited process 

 of concrescence (Fig. ^^). Once the germinal shield 

 is formed, the axis is to be considered as completed, and 

 no farther concrescence is possible except a slight migra- 

 tion inward of some of the adjoining regions of the germ 

 ring to form lateral regions of the embryo. There is no 

 opportunity for a median coalescing of outlying regions 

 of the germ ring to form any part of the embryonic axis. 



If this view is valid, it has a most important bearing 

 on our interpretation of conjoined twins in the fishes as 

 well as in other vertebrates. Such double-headed and 

 single-bodied individuals could not possibly arise from 

 a fusion of two separate embryonic shields, as Gemmill's 

 budding theory implies. In order to make such a fusion 

 possible it would be necessary to suppose that in some 

 extraordinary way the inner halves of each twin becomes 

 obliterated during the fusion process so that only the outer 

 halves remain, and that these outer halves come together 

 in such an extremely precise way as to fuse notochord 

 with notochord, neural tube with neural tube, aorta with 

 aorta, vein with vein, intestine with intestine. We also 

 would have to suppose that at the time of fusion the two 

 components were in exactly the same stage of develop- 

 ment, for otherwise the parts that come together in the 

 median line would not represent equivalent regions in 

 the primary axis, and there could be no exact equivalence 

 of contribution to the united posterior part of the body. 



In his later work Gemmill (191 2) accepts the inter- 

 pretation of Kopsch as to the formation of the embryonic 



